> Deveraj: > Jonathan Hsieh, WAL per region (WALpr) would give you the locality (and hence HDFS short > circuit) of reads if you were to couple it with the favored nodes. The cost is of course more WAL > files... In the current situation (no WALpr) it would create quite some traffic cross machine, no?
I think we all agree that wal per region isn't efficient on today's spinning hard drive world where we are limited to a relatively low budget or seeks (though may be better in the future with SSD's). With this in mind, I actually I making the case that we would group the all the regions from RS-A onto the same set of preferred regions servers. This way we only need to have one or two other RS's tailing the RS. So for example, if region X, Y and Z were on RS-A and its hlog, the shadow region memstores for X, Y, and Z would be assigned to the same one or two other RSs. Ideally this would be where the HLog files replicas have locality (helped by favored nodes/block affinity). Doing this, we hold the number of readers on the active hlogs to a constant number, do not add any new cross machine traffic (though tailing currently has costs on the NN). One inefficiency we have is that if there is a single log per RS, we end up reading all the logs to tables that may not have the shadow feature enabled. However, with HBase multi-wals coming, one strategy is to shard wals to a number on the order of the number of disks on a machine (12-24 these days). I think the a wal per namespaces (this could be used to have a wal per table) of the hlog would make sense. This way of shardind the hlog would reduce the amount of reading of irrelevant log entries on a log tailing scheme. It would have the added benefit of reducing the log splitting work reducing MTTR and allowing for recovery priorities if the primaries and shadows also go down. (this is an generalization of the separate out the META into a separate log idea). Jon. -- // Jonathan Hsieh (shay) // Software Engineer, Cloudera // j...@cloudera.com
